You're really late on finding this out, because Shugo Chara Egg! has been doing the openings since Shugo Chara! Doki! first began airing. Their first singe was Minna no Tamago and their second one, which is the one that's currently playing, is Shugo! Shugo!. Buono is still doing the ending songs, though.

I, personally, like Buono better as well. Shugo Chara Egg! have alright songs, but the animation and PV songs for them aren't that great. They tend to throw a much younger spin on the show. I wish Buono would go back to doing the opening for the duration of Ikuto's arc.